The present invention relates generally to a liquid crystal display (LCD), and more particularly, to an improved active matrix LCD.
In response to a rapidly increasing demand for space-saving, personalized displays which serve as the primary information transmission interface between humans and computers (and other types of computerized devices), various types of flat screen or flat panel displays have been developed to replace conventional display devices, particularly CRTs, which are relatively large, bulky, and obtrusive. Among the most attractive of these flat panel displays are LCDs, which, in some forms, match or surpass the color picture quality of CRTs. In particular, an LCD in an active matrix form utilizes a combination of liquid crystal technology and semiconductor technology, and is recognized as being superior to CRT displays.
The active matrix LCDs utilize an active device having a non-linear characteristic for driving each of a multiplicity of pixels arranged in a matrix configuration, to thereby provide both a memory function and an electronic optical effect of a liquid crystal. A thin film transistor (TFT) is ordinarily used as the active device. In an active matrix LCD which utilizes such active devices, millions or even billions of TFTs are integrated on a glass substrate together with a pixel address wiring pattern, to thereby provide an active matrix driver circuit, with the TFTs serving as switching elements. In such an active matrix LCD, the aperture ratio of the individual pixels is disadvantageously decreased, thereby concomitantly reducing the brightness of the LCD.
In order to overcome the above-described disadvantage, there has been proposed an improved active matrix LCD provided with an additional light shield layer, as described in an article entitled "High-Resolution 10.3-in.-Diagonal Multicolor TFT-LCD," by M. Tsunrura, M. Kitajima, K. Funahata, Y. Wakir, R. Saito, Y. Mikami, Y. Nagal, and T. Tsukada, in SID 91 DIGEST, pp. 215-218, published by Hitachi. In the improved active matrix LCD according to the above paper, a double light shield layer structure is employed in order to obtain a high aperture ratio and a high contrast ratio, thereby enhancing the visual display characteristics of the LCD. The double light shield layer structure includes a first light shield layer formed on a front glass substrate on which a conventional color filter is provided, and a second light shield layer formed on a rear glass substrate on which the TFTs are provided. An LCD having such a double light shield layer structure exhibits an aperture ratio which is improved by 6%-20% relative to an LCD which has only the conventional first light shield layer.
However, a significant drawback of the above-described LCD having a double light shield layer structure is that an opaque metal is used to form one of the electrodes of a storage capacitor associated with each pixel, thereby excluding the area occupied by each storage capacitor from the aperture area of its associated pixel, which has the effect of reducing the aperture ratio. Moreover, the process for fabricating the second light shield layer entails installing a light shield layer before forming an insulating layer merely for shielding light during the manufacturing of the TFTs, thereby necessitating additional process steps which unduly increase the cost and complexity of the LCD manufacturing process.
As is evident from the foregoing, there presently exists a need for an active matrix LCD which overcomes the above-described drawbacks and shortcomings of presently available active matrix LCDs. The present invention fulfills this need.